Dialogue between membranes and their lipid-metabolizing enzymes.

Biological chemists have often regarded the fluidity or plasticity of a membrane as a property that may be adaptively altered to accommodate changes in environmental temperature. The implication that cells exhibit an intelligent (or at least, appropriate) response provides us with several questions to examine. (1) Is temperature or some aspect of fluidity perceived by the responsive cellular element? (2) How is the detected difference from ‘ideal’ in temperature or fluidity communicated to the synthetic enzyme(s)? (3) How can the selective actions of the enzyme(s) of phospholipid synthesis be changed to alter the fatty acid content of membrane lipids? A primary intellectual challenge in examining these responses is to discern the degree to which the cells adapt because of signals from an inadequate membrane fluidity in contrast with an independent alteration of synthetic selectivity that coincidentally alters the membrane fluidity. In a sense, one might ask whether the intracellular signalling is a dialogue or a monologue, or in other terms, whether the manner of membrane adaptation is purposefully successful (‘Lamarckian’) or fortuitously beneficial (‘Darwinian’). Certainly cells containing either adaptive mechanism could exhibit better survival in a limiting environment than would those cells without. A complicating factor in evaluating adaptive changes in the fatty acid composition of membrane phospholipids is that the synthetic enzyme(s) may act on only single fatty acid molecules by recognizing their chemical structural features rather than fluidity, which is a co-operative aspect of the interaction of many molecules. Thus we must be sensitive to the language in which messages are translated and communicated within cells. Few of the enzymes handling fatty acid derivatives have shown the ability to discriminate appreciably among the different acyl chains, and selective actions seem likely for only the fatty acid synthases, desaturases and acyltransferases. The factors used by these enzymes in selecting preferred acyl chains seem important elements in the ‘language’ of membrane lipid adaptation.